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increase in the exogenous pO 2 from 4 to 40 kPa. Cultures grown at these pO 2 when transferred to
ambient pO 2 exhibited higher nitrogenase activity after 24 h of transfer. Walsby (2007) proposed that
the main gas diffusion pathway into the heterocysts is through the polar nodules that are present
at the junction of the heterocysts with vegetative cells. Here a third type of membrane (in addition
to cytoplasmic membrane and thylakoids) is found forming a honey-comb like structure (Lang
and Fay, 1971; Braun-Howland et al ., 1988) around the polar nodule with transmembrane proteins
making pores through which gas exchange can take place. For scavenging O 2 that enters through
these pores, the presence of respiratory enzymes in high concentration is a great possibility due to
the oxidation of diaminobenzidine (Murry et al ., 1981).
5) RESPIRATORY ACTIVITY
According to Walsby (1985) O 2 enters into heterocysts more slowly than N 2 but upon entry it is
immediately removed by a number of enzymes. The respiratory activity of heterocysts seems to serve
two functions, the fi rst is to scavenge O 2 from the site of nitrogen fi xation and the second is to generate
ATP in dark for supporting nitrogen fi xation. Fay and Walsby (1966) measured the rate of O 2 -uptake
by isolated intact heterocysts of A . cylindrica that was 40% more than the intact fi laments. However,
Bradley and Carr (1976) reported much lesser rates of O 2 -uptake. Nitrogen-fi xing cyanobacteria show
a high affi nity and low affi nity O 2 uptake systems. The high affi nity O 2 uptake system is associated
with growth in presence fi xed nitrogen sources whereas the low affi nity O 2 uptake system is operative
during nitrogen fi xation in dark. The low affi nity O 2 uptake is associated with an oxidase operative
in the heterocysts (Jensen and Cox, 1983). A characterization of mutants defective in heterocyst
envelope polysaccharides showed that these mutants lacked the low affi nity oxidase associated with
the heterocysts (Murry and Wolk, 1989). The generation of reduced cytochrome c-553 and donation
of electrons from cytochrome c-553 to either PSI or respiratory cytochrome oxidases in vegetative
cells (Peschek, 1987) or heterocysts (Böhme and Almon, 1983; Böhme and Ernst, 1984; Houchins and
Hind, 1983; Houchins, 1984 1985) has been demonstrated in A . variabilis ATCC 29413 or Anabaena
sp. strain PCC 7120. Wastyn et al . (1988) reported that the rates of cytochrome c-553 oxidation were
far greater by the cytoplasmic membrane and thylakoids from heterocysts of A . variabilis ATCC
29413 than by the corresponding membranes from vegetative cells. Likewise, Houchins and Hind
(1984) found that the level of cytochrome aa3 in heterocyst membranes was 4 and 100 times higher
than in the vegetative cells of Anabaena sp. strain PCC 7120. The observations of Wastyn (1988) on
immunoblotting experiments as well as those of Häfele et al . (1988) on other biochemical properties
have brought to light that the oxidase is cytochrome oxidase aa3 type, a mitochondrial-type oxidase
that is unique for cyanobacteria. An operon coding the subunits I, II and III of cytochrome aa3 has
been cloned from Synechococcus vulcanus (Tano et al ., 1991), Synechocystis sp. strain PCC 6803 (Alge
and Peschek, 1993; Alge et al ., 1999) and A . variabilis ATCC 29413 (Schmetterer et al ., 2001). Jones
and Haselkorn (2002) cloned and sequenced two operons of cytochrome oxidase ( cox ) of aa3 -type,
coxBACI and coxBACII encoding three subunits in each case. Of these, the latter was induced more
than 20-fold at middle to late stages (12 to 14 h) of heterocyst differentiation in Anabaena sp. strain
PCC 7120. While the expression of coxBACII was restricted to the proheterocysts and heterocysts,
the expression of coxBACI was found in all cells when grown with nitrate or ammonium. Further, it
nominally increased after a nitrogen step-down that was followed by a gradual decline afterwards.
These observations were confi rmed with the use of gfp - met2 as a reporter gene with coxBACII and
coxBACI promoters in the wild-type Anabaena sp. strain PCC 7120. The coxBACI operon of Anabaena
sp. strain PCC 7120 is an orthologue of A. variabilis ATCC 29413 reported earlier by Schmetterer et
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